Hydrophilic/hydrophobic patterned surfaces and methods of making and using the same

ABSTRACT

One embodiment includes a substrate having a plurality of molecular chains, each chain comprising a hydrophilic group, a hydrophobic segment, and a reversible crosslinker.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 12/119,652filed May 13, 2008 which claims the benefit of U.S. ProvisionalApplication No. 60/939,674, filed May 23, 2007.

TECHNICAL FIELD

The field to which the disclosure generally relates includeshydrophilic/hydrophobic patterned surfaces, products including the sameand methods of making and using the same.

BACKGROUND

Products having hydrophilic or hydrophobic surfaces have broadapplications running from proton absorption, water transport (e.g. fuelcells), friction control, etc. On a superhydrophobic surface, waterstays in a droplet form and rolls off easily when the surface isslightly tilted. Achieving superhydrophobicity usually requires textureon the surface such as the well-known lotus leaves. On a typical flathydrophobic surface, a water contact angle of as high as 110° can beachieved.

Water, when dropped on a superhydrophilic surface (water contact anglebelow 20 degree), spreads simultaneously in a radial fashion. If thesuperhydrophilic surface contains micro-channel features, water wicksinto the channels and moves quickly along the channels due to theadditional capillary force, with the velocity depending on the channelsize. This water wicking behavior has been found particularly useful infacilitating water transport in micro-channels.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

One embodiment of the invention includes a substrate having molecularchains attached thereto, each chain comprising a hydrophilic group, ahydrophobic segment, and a photo-reversible crosslinker.

Another embodiment of the invention includes a substrate having areconstructable hydrophobic/hydrophilic patterned surface comprising aplurality of molecular chains, each chain containing a hydrophilicgroup, a hydrophobic segment and a photo-reversible crosslinked portion.

One embodiment of the invention includes a method including providing asubstrate having a plurality of molecular chains attached thereto, eachchain comprising a hydrophilic group, a hydrophobic segment, and areversible linker, and causing the reversible linker to crosslinkadjacent molecular chains.

Another embodiment of the invention comprises providing a substratehaving a plurality of molecular chains attached thereto, each chaincomprising a hydrophilic group, a hydrophobic segment, and aphoto-reversible crosslinker, exposing the substrate to a charge andthereafter exposing the molecular chains to ultraviolet light tocrosslink adjacent molecular chains. In one embodiment of the invention,the substrate is exposed to a positive charge, causing the molecularchain to straighten and so that the hydrophilic group is furthest fromthe substrate. In another embodiment, the substrate is subjected to anegative charge causing the molecular chains to bend over so that thehydrophilic group is attached to the substrate.

Another embodiment of the invention includes providing a substratehaving a plurality of molecular chains, each chain containing ahydrophilic group and a hydrophobic segment and wherein adjacent chainsare connected together by a reversible crosslinked portion, and causingthe crosslinked adjacent molecular chains to be uncrosslinked byexposing the chains to ultraviolet light. In one embodiment of theinvention, the crosslinked chains are exposed to ultraviolet light of awavelength less than 260 nm.

Other exemplary embodiments of the invention will become apparent fromthe detailed description provided hereinafter. It should be understoodthat the detailed description and specific examples, while disclosingexemplary embodiments of the invention, are intended for purposes ofillustration only and are not intended to limit the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will become more fullyunderstood from the detailed description and the accompanying drawings,wherein:

FIG. 1 illustrates a method of forming hydrophilic surfaces andhydrophobic surfaces on a substrate.

FIG. 2 illustrates a method of surface modification of a substrateaccording to one embodiment of the invention.

FIG. 3 illustrates a product according to one embodiment of theinvention.

FIG. 4 illustrates a product according to another embodiment of theinvention.

FIG. 5 illustrates a prior art fuel cell bipolar plate with amicrogroove.

FIG. 6 illustrates a product according to another embodiment of theinvention.

FIG. 7 illustrates a product according to another embodiment of theinvention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description of the embodiment(s) is merely exemplary innature and is in no way intended to limit the invention, itsapplication, or uses.

One embodiment of the invention includes a channel-less fluidic device.In one embodiment, a surface with both hydrophobic and hydrophilic microor nano-patterns (e.g. strips) is created through careful moleculardesign. By selectively forming hydrophilic and hydrophobic patterns onthe surface, wherein in some cases the contact angle difference can beas large as 100°, water dropped onto such a pattern surface is expectedto behave as a micro-channeled superhydrophilic surface with the borderbetween the hydrophobic and superhydrophilic area acting as a virtualchannel wall. In one embodiment, the molecular design can make thevirtual water channel reconstructable, as described hereafter.

As shown in FIG. 1, one embodiment of the invention includes a substrate10 modified with a plurality of molecular chains 12, with each chain 12including a hydrophilic group 18, a hydrophobic segment 14, and areversible cross linker portion 16. In one embodiment, to createhydrophobicity, fluorocarbon chains may be used as the hydrophobicsegments 14. Hydrophilicity may be created with polar groups 18. In oneembodiment, the hydrophilic group 18 may be a carboxylic acid (or salt)group. In one embodiment, the polar groups are ionic groups andintroduce superhydrophilicity. In one embodiment, adjacent molecularchains 12 are crosslinked together which tends to stabilize surfacesuperhydrophilicity unlike uncrosslinked superhydrophilic surfaces whichare prone to hydrophilicity loss over time when exposed to air. In oneembodiment, the reversible cross linker portion 16 may be a cinnamicacid link.

Referring now to FIG. 2, in one embodiment of the invention, a substrate10 may be created, for example, by the copolymerization of vinyl acetatewith a small amount of divinyl benzene used as a curing agent.Hydrolysis occurs on a surface and generates hydroxyl groups, whichreact further with perfluoro dicarbonyl chloride. The perfluorodicarbonyl chloride may be used in large excess to avoiddiesterification. Phenol groups are generated in the next step throughthe reaction between carbonyl chloride groups and an excessive amount ofdry hydroquoine. In the final step, cinnamic acid structural units areintroduced onto the surface through the phenol groups. This chemistryproduces a surface that contains fluorocarbon chains with hydrophiliccarboxylic acid or salt end groups, linked together by crosslinkabledouble bonds.

Referring again to FIG. 1, by imposing positive charges on the substrate10 through a DC e-field or soaking the substrate in a hydrophilicsolvent, the molecular chains 12 on the substrate surface are stretchedwith the ionic groups 18 pointed away from the surface. The sample maybe exposed to ultraviolet light of wavelength greater than 260 nm topromote UV curing through the photo-dimerization of cinnamic acidlinkages. The end result is a crosslinked superhydrophilic surface 22with ionic groups 18 on top.

Also as illustrated in FIG. 1, when a negative charge is imposed on thesubstrate 10 or a hydrophobic solvent is used, the opposite occurs. Thatis, the chains 12 bend over so that the ionic group 18 attaches to thesubstrate 10. This produces a hydrophobic surface 24 dominated byfluorocarbon segments 14 of the chains 12 which is expected to have acontact angle of about 110°.

The invention is not limited to the specific arrangement of components14, 16, and 18 of the molecular chain 12 as shown in FIG. 1. Forexample, the hydrophilic component 18 need not be located at the end ofthe molecular chain 12 nor does the crosslinkable portion 16 need to belocated adjacent the hydrophilic component 18. Further, additionalsegments or groups may be interposed between components of the molecularchain 12 or attached to the ends thereof. For example, an additionalgroup or segment may be attached to the hydrophobic segment 14 to attachor anchor the molecular chain 12 to the substrate 10.

Due to spatial selectivity of ultraviolet reactions, patterns ofhydrophobicity and superhydrophilicity can be created in different areasof the same substrate by sequentially employing the above-described twoprocesses and the use of a photomask on opposite patterns. The featuresize of the patterns can be as low as nanometers due to the excellentspatial resolution of ultraviolet wavelengths.

The crosslinking reaction can also be reversed by exposing the patternedsurface to ultraviolet light of wavelengths less than 260 nm,characteristic of cinnamic acid based systems. When the cross linkingbonds are cleaved, the surface returns to its original state and thesuperhydrophilicity and hydrophobicity patterns are erased. By repeatingthe two processes shown in FIG. 1 using a mask on different features,new patterns of superhydrophilicity and hydrophobicity can be recreated.

Referring now to FIG. 3, another embodiment of the invention includes afuel cell stack 100 including a substrate 10 that may be a first fuelcell bipolar plate 112. The surface of the first bipolar plate 112 maybe modified to include a pattern of hydrophilic 22 and hydrophobicregions 24. Optionally, the first bipolar plate 112 may include channels90 formed in an opposite face for flow a coolant therethrough. A firstgas diffusion media layer 76 may underlie the first bipolar plate 112.The first gas diffusion media layer 76 is porous and typical formed ofcarbon papers arranged as carbon paper or felt. A first microporouslayer 72 may be provided underlying the first gas diffusion layer 76.The first microporous layer 72 may include carbon particles and PTFE. Acathode 68 may underlie the first microporous layer 72 and may include acatalyst such as platinum on carbon particles and an ionomer such asNAFION. A polyelectrolyte membrane (PEM) 62 may be provided underlyingthe cathode and may include an ionomer such as NAFION. An anode 70 mayunderlie the membrane 62 and may be similarly constructed as the cathode68. A second microporous layer 74 may underlie the anode 70 and a secondgas diffusion media layer 78 may underlie the second micorporous layer74. A second biopolar plate 114 may underlie the second gas diffusionmedia layer 78. In the embodiment shown in FIG. 3, reactant gases areeach respectively forced through the respective gas diffusion medialayers 76, 78. Any liquid water present due to condensation ofhumidified gases or created as a byproduct of the fuel cell reaction maywick along the hydrophilic regions 22 of the surface of each bipolarplate 112, 114 and be removed from the cell if desired.

Referring now to FIG. 4, the second bipolar plate 114 may have at leasttwo lands 26 and one channel (gap or space) 28 formed between the lands.The channel 28 may be defined by walls 27 of the land 26 and a floorsurface 29. The floor surface may be modified to include a pattern ofhydrophilic 22 and hydrophobic regions 24. The lands 26 make physicaland electrical contact with the second diffusion media layer 78. Areactant gas, such as but not limited to hydrogen, flows through thechannel 28, and diffuses through the gas diffusion layer 78 to theanode. Any liquid water present due to condensation of humidified gasesor created as a byproduct of the fuel cell react may wick along thehydrophilic regions 22 of the surface of each bipolar plate 112, 114 andbe removed from the cell if desired. The gas flowing in the channel 28may help to push the water out of the cell.

FIG. 5 illustrates a prior art fuel cell bipolar plate with amicrogroove 50 formed in the floor 29 of the surfaces defining thechannel 28. According to one embodiment, as shown in FIG. 6, one or morevirtual microgrooves may be formed along the floor 29 or walls 27defining the channel 28 by providing a region 22 comprising a polarityof crosslinked molecular chains comprising a hydrophilic group orsegment. As shown in FIG. 7, the substrate 10 may be a metal such asstainless steel, and a polymeric coating 52 may be provided in thechannel 28 along a portion of the wall 27 and/or floor 29. A region 22comprising a plurality of crosslinked molecular chains comprising ahydrophilic group or segment may be provided on the coating 52. Themolecular chains may be a part of the coating 52 or attached thereto. Inone embodiment the width of the region 22 defining the virtualmicrogroove may be less than 50 μm. The channel may have a variety ofconfigurations and may also be semicircular or V-shaped.

The above description of embodiments of the invention is merelyexemplary in nature and, thus, variations thereof are not to be regardedas a departure from the spirit and scope of the invention.

What is claimed is:
 1. A product comprising a substrate comprising aplurality of molecular chains wherein each chain comprising ahydrophilic group, a hydrophobic segment, and a reversible crosslinkerconstructed and arranged to reversibly crosslink adjacent molecularchains to change a portion of the substrate from hydrophilic tohydrophobic or from hydrophobic to hydrophilic.
 2. A product comprisinga substrate, a plurality of molecular chains attached to the substrate,each chain comprising a hydrophilic group, a hydrophobic segment, and aphoto-reversible crosslinker constructed and arranged to reversiblycrosslink adjacent molecular chains to change a portion of the substratefrom hydrophilic to hydrophobic or from hydrophobic to hydrophilic.
 3. Aproduct comprising a substrate having a reconstructablehydrophobic/hydrophilic patterned surface comprising a plurality ofmolecular chains, each chain containing a hydrophilic group, ahydrophobic segment and a photo-reversible crosslinked portionconstructed and arranged to reversibly crosslink adjacent molecularchains to change a portion of the substrate from hydrophilic tohydrophobic or from hydrophobic to hydrophilic.
 4. A product comprisinga fuel cell bipolar plate comprising a substrate comprising a patternedsurface comprising hydrophobic and hydrophilic regions comprising aplurality of molecular chains, each chain containing a hydrophilicgroup, a hydrophobic segment and a reversible crosslinked portioncrosslinker constructed and arranged to reversibly crosslink adjacentmolecular chains to change a portion of the surface from hydrophilic tohydrophobic or from hydrophobic to hydrophilic.
 5. A product as setforth in claim 4 wherein chains in the hydrophilic region are stretchedso that the hydrophilic group is furthest from the substrate to providea surface of the hydrophilic groups.
 6. A product as set forth in claim4 wherein the chains in the hydrophilic group in the hydrophilic regionare bent so that the hydrophilic group is attached to the substrate anda surface of the hydrophobic segment is provided.
 7. A product as setforth in claim 4 wherein the bipolar plate does not include a reactantflow field defined by lands and channels therebetween.
 8. A product asset forth in claim 4 wherein the bipolar plate comprising a reactant gasflow field is defined by at least two lands and a channel therebetween,the channel being defined at least in part by a floor surface andwherein the patterned surface is on the floor surface.
 9. A productcomprising a fuel cell bipolar plate having a reactant gas flow fielddefined therein by at least a plurality of lands and channels, thechannel being at least partially defined by a surface, and a virtualmicrogroove over the surface comprising a plurality of molecular chainseach comprising a hydrophilic group and a crosslinked portioncrosslinker constructed and arranged to reversibly crosslink adjacentmolecular chains to change a portion of the surface from hydrophilic tohydrophobic or from hydrophobic to hydrophilic.
 10. A product as setforth in claim 9 wherein each molecular chain further comprises ahydrophobic segment.
 11. A product as set forth in claim 9 wherein thecrosslinked portion is reversibly crosslinked.
 12. A product as setforth in claim 9 wherein the region defining the virtual microgroove hasa width less than 50 μm.
 13. A product as set forth in claim 1 whereineach chain is bent over and wherein the hydrophilic or another group isattached to the substrate.
 14. A product as set forth in claim 1 whereineach chain is bent over and wherein the hydrophilic group is attached tothe substrate.
 15. A product as set forth in claim 2 wherein each chainis bent over and wherein the hydrophilic or another group is attached tothe substrate.
 16. A product as set forth in claim 2 wherein each chainis bent over and wherein the hydrophilic group is attached to thesubstrate.
 17. A product as set forth in claim 3 wherein each chain isbent over and wherein the hydrophilic or another group is attached tothe substrate.
 18. A product as set forth in claim 3 wherein each chainis bent over and wherein the hydrophilic group is attached to thesubstrate.
 19. A product as set forth in claim 4 wherein each chain isbent over and wherein the hydrophilic or another group is attached tothe substrate.
 20. A product as set forth in claim 4 wherein each chainis bent over and wherein the hydrophilic group is attached to thesubstrate.